October 12, 191 1] 



NATURE 



48- 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Simultaneity of Certain Abruptly-beginning 

 Magnetic Disturbances. 



In the first volume of my work " The Norwegian Aurora 

 Polaris Expedition, 1902-3," I stated (p. 63), after study- 

 ing the magnetograms from seventeen stations in all 

 quarters of the earth, that the characteristic sudden similar 

 magnetic changes occurring often during positive equatorial 

 perturbations " appear simultaneously, or rather, the 

 differences in time are less than the amount that can be 

 detected by these registerings. " " The above question on 

 simultaneity, which is of great importance, cannot be 

 •definitely decided until we are in possession of rapid 

 registerings." 



In the spring of 1911 I made an investigation of the 

 Eodiacal light in the Sudan and Egypt, during which I had 

 mounted two complete sets of registering apparatus, the 



Eanohihi 



together with Aswan (.33° E.), form a particularly happy 

 distribution of stations about the earth. 



The figure shows that on this day an equatorial per- 

 turbation occurred the character of which on these three 

 stations is very similar. The times of commencement in 

 H are as follows : — 



Honolulu Porto Rico Aswan 



ion. 20m. 7s. p.m. G.M.T. ... 20m. Ss. ... 20m. 44s. 



The changes in D at the same time were very small, as 

 might be expected would be the case with this kind of 

 perturbation. 



The first notices of time are given in a letter from the 

 Coast and Geodetic Survey ; the last value is found by the 

 quick-run magnetogram from Aswan, as shown by the 

 diagram. The time-marks here, which refer to the central 

 point for the obliterated parts, are certainly correct to one 

 second, but a greater uncertainty arises when it is a ques- 

 tion of determining when the perturbation shall be said 

 to have commenced. I consider we may be safe when we 

 estimate the possible error at +4 seconds. But the values 

 of the slow-run magnetograms lie within this margin in 

 Honolulu and Porto Rico, where, however, the readings 

 are naturally not so trustworthy as those of the quick-run 

 magnetograms from Aswan. 



Tenia Bico 



9Ajir 1!)11 



one for slow, the other for rapid, registrations, which were 

 working every night near Aswan for one month under the 

 supervision of Mr. Krogness. 



The instruments were set up in the depth of an ancient 

 Egyptian tomb, in which the temperature was fairly 

 ■constant. Thanks mainly to Mr. Keeling, the super- 

 intendent of the Khedivial Observatory in Helwan, we 

 enjoyed all the facilities for our work that we could desire. 



In order to obtain " quick-run " magnetograms from 

 ■other stations taken at the same time, I published in 

 Nature for March 16 (No. 2159, p. 79) a letter requesting 

 ■other observatories to take such registrations at the same 

 hours as we. I have, unfortunately, not received any 

 intimation of any such " quick-run " registrations having 

 been taken except in Samoa, where Dr. Augenheister com- 

 menced the registrations on April 10, while the onlv sudden 

 equatorial storm registered in the prescribed period of one 

 month occurred on April 9. 



Mr. Tittmann, superintendent of the U.S. Coast and 

 Geodetic Survey, has been good enough to send me some 

 copies of slow-run registerings for April o from Chelten- 

 ham, Porto Rico, Tucson, Sitka, and Honolulu. Of these, 

 the curves from Honolulu (158° W.) and Porto Rico 

 (65° W.) are of special interest, because these stations, 

 NO. 2189, VOL. 87] 



The curves from Sitka, Tucson, and Cheltenham show , 

 that the perturbations in those places have had a somewhat 

 different character from those at the three first-named 

 stations, for it appears as though a magnetic polar storm 

 interferes. The curves for D and Z show the same 

 thing. 



The times we have been given from the Coast and 

 Geodetic Survey for these stations are : — for Sitka, 

 ioh. 2irn. ; Tucson, loh. 20m.; and Cheltenham, 

 ioh. 21m. 9s.; and these refer to the " larger displace- 

 ment " in H. This occurs shortly after the first abrupt 

 beginning, and the times are, as may be seen, with the 

 exception of Tucson, slightly greater than the others. 



As regards Tucson, we notice that the first time-mark 

 is considerably smaller than the later ones ; for this reason 

 I think this value perhaps ought to be taken with some 

 reservation. 



From Trondhjem I have only one slow-run magneto- 

 gram, as the corresponding quick-run, unfortunately, is 

 wanting for the only hour in question. At this station 

 the polar character "I the storm is distinctly apparent, as 

 might be expected from so high a latitude. On the occasion 

 of the magnetic storm we are h^re studying, the similar 

 sudden changes occurred thus around the terrestrial equator 



